Perpetual motion machines are machines that produce an unlimited supply of energy, or just never stop. Inventors have tried for centuries to find a working perpetual motion machine, but all have failed, sometimes producing novel ideas, though. It can be shown using the First (conservation of energy) and Second (entropy can never decrease) )Laws of Thermodynamics that any perpetual motion machine is impossible, even though people still try in vain to find them.
Most perpetual motion machines are based on a simple machine, the overbalanced wheel, which was invented during or before the Middle Ages. The idea is that the weighted rods are on hinges so that they stay close to the wheel while going up, and stick out when going down on the wheel, so that the right side will always have more weight, and the left side will always have less, and so the wheel, once started, should never stop.